Hook up Rear Speakers?

I just bought a 2006 MX-5 that came with the standard multi-function stereo. I noticed there are two small grills between the two seats in the rear that look like they may have been for the Bose stereo speakers. When I turn the fader to the rear, no sound comes out. Does anyone know if a couple of speakers can be added and will work with the multi-function radio? I assume since there is a fader in the radio there must be a place to connect additional speakers. Thanks.